It occurs immature in the gastropod Nassa.
The problem of the causes of the torsion of the Gastropod body has been much discussed.
B, The same, partially everted by eversion of the sides, as in the Nemertine proboscis and Gastropod eye-tentacle = pleurecbolic.
Walcott has described a tiny lamellibranch (Modioloides) from the inferior Cambrian, and he reports the gastropod (?) genus Chuaria from the preCambrian.
According to Spengel, the pair of ganglia near the mouth, variously called labial or cerebral, represent the cerebral pair and pleural pair of a gastropod combined, and the parietosplanchnic pair correspond to the visceral ganglia, the commissure which connects them with the cerebro-pleural representing the visceral commissure.
